The invention relates to methods which permit the reduction or elimination of localized states in the energy gap, such as dangling bonds, recombination centers, etc. in amorphous semiconductor films to provide improved amorphous semiconductor films which have characteristics like those found in corresponding crystalline semiconductors. The amorphous films involved have their most important utility in solar radiation energy-producing devices, and current control devices, such as p-n junction devices including rectifiers, transistors or the like, where heretofore crystalline semiconductor bodies have been used in their fabrication.
The principles involved in the invention can be applied to various types of amorphous semiconductor films, both thick and thin films, which have recombination centers and other localized states inhibiting the control of the conductivity thereof, and are applicable to amorphous semiconductor films made of one or more elements, or combinations of elements which are mixtures or alloys of such elements. Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells.
Since amorphous silicon-containing films, if made equivalent to crystalline silicon films, would have many advantages over such crystalline silicon films (e.g. lower cost, larger area, easier and faster manufacture), the main purpose of this invention is to overcome the barrier which has heretofore prevented materials such as amorphous silicon from having characteristics similar to crystalline silicon. Since this invention has overcome what up until now has been an impenetrable barrier to producing useful amorphous silicon films, we therefore initially deal with silicon films, although many aspects of the invention are also applicable to the production of films of various other amorphous semiconductor materials formed by elements including individual elements or mixtures or alloys of elements falling in Groups III through VI of the periodic table.
When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially impurity-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p and n conductivity regions therein. This was accomplished by diffusing into such pure crystalline materials parts per million of donor or acceptor dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction and photoconductive crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants. These crystal growing processes produce such relatively small crystals that solar cells require the assembly of hundreds of single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up such a crystalline material has all resulted in an impossible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p and n junctions where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short diffusion lengths, making such films unsuitable for solar cell applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in the amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping said amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them more extrinsic and of p or n conduction types. This was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) and a gas of phosphine (PH.sub.3) for n-type conduction, or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction, were premixed and passed through a reaction tube where the gaseous mixture was decomposed by an r.f. glow discharge and deposited on a substrate at a high substrate temperature of about 500.degree.-600.degree. K. The material so deposited on the substrate is an amorphous material consisting of silicon and hydrogen and substitutional phosphorus or boron in dopant concentrations between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. However, the electrical characteristics achieved by doping these materials did not reach the levels which make them commercially acceptable devices, such as solar cell devices, and current control devices including effective p-n junction devices and the like. The major scientific problem remained, i.e., the remaining states in the gap could not be eliminated.
As expressed above, amorphous silicon, and also germanium, is normally four-fold coordinated, and normally has microvoids and dangling bonds, producing localized states in the energy gap. While it is believed that it was not known by these researchers, it is now known that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to decrease substantially the density of the localized states in the energy gap toward the end of making the amorphous material approximate more nearly the corresponding crystalline material. However, the effect of the hydrogen was limited by the fixed ratio of hydrogen and silicon in silane as well as limiting the type of hydrogen bonding and introducing new anti-bonding states all of which can be of importance in these materials. Therefore, as above indicated, the density of the localized states was not reduced sufficiently to render these films commercially useful in solar cell or current control devices.
In addition to the limitations described above, the silane glow discharge deposition of silicon film poses problems which further hinder its commercial suitability. For example, such a process does not lend itself to the mass production of amorphous semiconductor films because it is a slow process, difficult to control and requires silane which is a relatively expensive starting material.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in an atmosphere of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the molecular hydrogen acted somewhat as a compensating agent to reduce the localized states of the energy gap. However, the degree to which the localized states of the energy gap were reduced in the sputter depositing process was too minimal to be useful for commercial purposes. The degree of reduction in the density of localized states achieved by this sputter deposition process was much less than that achieved by the silane deposition process described above, as would be expected since sputter and vapor deposition processes inherently produce amorphous films with much higher densities of localized states than does a glow discharge deposition process. This is the reason that prior to the present invention, it was not believed that sputter or vapor deposition processes could successfully produce amorphous semiconductor films functionally equivalent to similar crystalline materials used in solar cell and current control devices. Also, the sputtering process must be carried out under certain critical partial pressure limitations, and since such partial pressures are effected both by the amount of argon and hydrogen gas present, the amount of molecular hydrogen gas which could be introduced into the sputtering atmosphere was accordingly limited.
The difficulty encountered heretofore in reducing the density of localized states in the energy gap of amorphous semiconductor materials like silicon and others to desirably low levels, so that these materials are the equivalent of corresponding crystalline materials, is believed to be explainable in the following manner. At or near the Fermi level of these materials deposited, for example, by the glow discharge of silane, are two bumps of relatively high density of states in the energy gap which are apparently related to the remaining dangling bond density. They are located substantially at about 0.4 eV below the conduction band E.sub.c and above the valence band E.sub.v. When the glow discharge amorphous silicon is doped with phosphorus or boron, the Fermi level is believed to be moved up or down, but the density of localized states was so high that the dopant could not move the Fermi level close enough to the conduction or valence bands to have an effective p or n junction. Thus, the activation energy for the doped glow discharge amorphous silicon was not lowered below about 0.2 eV. This result also placed a theoretical limitation on the open-circuit photovoltage of a p-n junction of doped glow discharge amorphous silicon, since the internal field cannot exceed the separation of the Fermi level in the p and n type regions. In addition, the remaining activation energy limits the room-temperature DC conduction of the doped glow discharge amorphous silicon and the material would have a large sheet resistance if it were made into a large area array, the resistance not being helped by the rather low carrier mobility which is a factor of about 10.sup.4 -10.sup.5 less than that for crystalline silicon. Also, where it is desirable to modify an amorphous silicon film to form an effective ohmic interface, for example, between an intrinsic (undoped) portion thereof and an outer metal electrode, such modified portions of the film must have a very high conductivity. It is apparent that the prior methods of doping such films which produced a conductivity of only 10.sup.-2 (ohm cm).sup.-1 would not provide a useful ohmic interface.
As discussed, the prior deposition of amorphous silicon, which has been compensated by hydrogen from the silane in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, all done during the glow discharge deposition, has characteristics which in all important respects are inferior to those of doped crystalline silicon and cannot be used successfully in place of doped crystalline silicon.
In contrast, the present invention enables the amorphous silicon and other films to be modified by the addition of conduction-increasing materials so that the conductivity is increased to approximately one (ohm cm).sup.-1 or greater, which makes the films useful as ohmic interfaces between other portions of the films and metal electrodes, as well as solving the basic problems of full compensation of multi (various) recombination sites and, therefore, the creation of materials able to form efficient and effective p-n junctions.